Such a filtering device is known for instance from DE 10 2005 022 138 A1. This filtering device is characterized in that it is particularly suited for filtering out dust, dioxins, nitrogen oxides or the like, in particular from blast-furnace waste gasses or waste-incinerating plants. For this purpose, it has a number of inclined beds which are provided with a filling of catalytic material. At the lower longer side of the bed, the catalytic material borders a retaining wall each, and the bed is penetrated by a flow or current from top to bottom in order to achieve the desired filtering effect.
If the catalytic material is clogged, in particular due to the resulting dust, a flush gas is supplied through the bed from bottom to top. As a result, the uppermost and compressed layer of catalytic material is slightly lifted and will slide off over the retaining wall.
Obviously the angle of repose of the filling material must correspond to the angle of inclination of the bed in order to secure an optimal operation.
For industrial use, such filtering devices are of considerable lengths. For example, a bed can be six meters or even ten meters long in the longitudinal direction, with the width then amounting to one meter, for instance. The bed which is inclined, perceived in a cross direction, is then underflushed over its entire length, i.e. over six or ten meters, by the flush gas, and the flush gas penetrates gradually through the bed from underneath.
In order to guarantee a safe lifting of the compression layer, it has been suggested to work with flush-gas impulses. This results in a slightly more even removal of the compression layer, but the pressure wave thus created will cause also catalytic material which is not yet worn out to be catapulted off, such that the resulting losses are too high.
In order to improve the cleaning of the filling, it has also been suggested to attach flush-gas entrances laterally, i.e. along the lower longer side of the bed, for instance. However, this is unfavourable in the process because such entrances are then easily polluted by the compression layer sliding downwards, and if the flush-gas impulse is kept very short, such that no flush gas is flowing any more when the compression layer slides down, parts of the compressed granules will inevitably enter the flush-gas apertures. In addition, a large number of flush-gas pipe connections at the outer side of the filter box makes it necessary to have a corresponding number of sealings, pipe diversions and branchings and is insofar unfavourable to put into practice.